Acidic and basic fibroblast growth factors (aFGF and bFGF) are mitogenic peptides which also have angiogenic, neurotropic, and morphogenic activities. They have been found in most tissues but their cellular origin in vivo is unknown. Approximately half of all mesenchymal cells synthesize bFGf in culture and a minority of these cells synthesize aFGF in culture. However, the in vitro expression may not be a true indicator of the situation in vivo. The absence of immunocytochemistry or in situ hybridization for these peptides and genes, respectively, has prevented their cellular localization. We took another approach and isolated cardiac myocytes by enzymatic digestion. These cells were then washed, lysed and subjected to heparin sepharose chromatography. The fractions which elute in the positions characteristic of aFGF and bFGF were then tested for mitogenic activity on 3T3 cells and tested for immunoreactivity for acidic and basic FGF by dot blot. Both types of assays suggest that these peptides are abundant in cardiac myocytes. The roles of these peptides in young adult rat cardiac myocytes, which are not dividing and not subject to pathologic hypertrophy or to angiogenesis, will require further investigation.